Cold rolling mill

ABSTRACT

The present application describes a Cold Rolling Mill (CRM) 200. The CRM 200 comprises a pair of working rolls 202 configured to apply stress on a metal strip for reducing thickness of the metal strip. The pair of working rolls 202 have a face width of 1350 mm. The CRM 200 further comprises a pair of intermediate rolls 204 configured to provide mechanical support to the pair of working rolls 202. The pair of intermediate rolls 204 have a face width of 1280 mm. The CRM 200 further comprises a pair of back-up rolls 206 configured to provide mechanical support to the pair of intermediate rolls 204. The back-up rolls 206 have a face width of 1300 mm. Bearing center distance of the CRM 200 is 2170 mm.

FIELD OF INVENTION

The present invention generally relates to a Cold Rolling Mill (CRM). More specifically, the present invention is related to a 4-Hi CRM and a 6-Hi CRM.

BACKGROUND OF THE INVENTION

The subject matter discussed in the background section should not be assumed to be prior art merely as a result of its mention in the background section. Similarly, a problem mentioned in the background section or associated with the subject matter of the background section should not be assumed to have been previously recognized in the prior art. The subject matter in the background section merely represents different approaches, which in and of themselves may also correspond to implementations of the claimed technology.

Cold Rolling Mills (CRMs) are used to reduce thickness of metal strips by pressing them at room temperature. The metal strips are pressed by passing them through a pair of rolls called work rolls. The pair of work rolls are often supported by one or more pairs of supporting rolls, and based on the number of pairs of supporting rolls that are utilized, the CRM are identified as 4-Hi CRM, 6-Hi CRM, 12-Hi CRM, and so on. To obtain a desired thickness of the metal strips, the metal strips are passed through the working rolls for a predefined number of times.

Referring now to FIG. 1 illustrating a side view of a CRM 100 pressing a metal strip 102, the process of cold rolling is now explained. The metal strip 102 is passed through a pair of work rolls 104, i.e., top work roll 104-1 and bottom work roll 104-2. The pair of work rolls 104 is supported by a pair of supporting rolls 106, i.e., top supporting roll 106-1 and bottom supporting roll 106-2. The metal strip 102 having a higher thickness is fed between the pair of work rolls 104 and the metal strip 102 having a lower thickness leaves the pair of work rolls 104. Because stress is applied onto the metal strip 102 by the pair of work rolls 104 and tension is applied along length of the metal strip 102, thickness of the metal strip 102 reduces and length increases after the rolling operation.

While imparting the stress, the pair of work rolls 104 makes contact with a specific region of the metal strip 102, and such region is called arc of contact 108. During an operational cycle, friction is created and plastic deformation occurs, in the arc of contact 108, while contact occurs between the pair of work rolls 104 that are made of a hard material and the metal strip 102 that is made of a relatively soft material. Due to the friction caused by relative sliding between the work rolls 104 and the metal strip 102 and plastic deformation of the metal strip 102, significant amount of heat is generated around the arc of contact 108. Additionally, heat transfer also occurs from the metal strip 102 to the work rolls 104 because of temperature difference.

The heat will continuously migrate to cooler zones in the work rolls 104 and out of body of the work rolls 104, at localized chill zones created by the coolant sprays 110 (part of a heat exchanger) impingement on the roll surface. The work rolls 104 are subject to thermal fatigue and mechanical fatigue during rolling operations. Thermal fatigue occurs as the work rolls 104 cycle through elevated temperature in the rolling operation and lower temperature zones are cooled under the coolant sprays 110. Mechanical fatigue occurs by mechanical compression (flattening) and physical deflection caused by rolling force and torque applied by a motor running the work rolls 104. Also, if the metal strip 102 is very thin, the top work roll 104-1 and the bottom work roll 104-2 may contact each other beyond the edges of the metal strip 102.

Thus, there remains a need of an improved design of (CRMs) that do not require heat exchangers, have better reduction capability, do not suffer from thermal fatigue and mechanical fatigue, reduces electrical power requirements for running the CRMs, provides metals strips of better shape, and achieve desired thickness in least number of passes.

OBJECTS OF THE INVENTION

A general objective of the invention is to provide a CRM that provides an increased transfer of stress on a metal sheet or strip in contact with a pair of working rolls.

Another objective of the invention is to provide a CRM that requires a reduced number of passes/iterations for pressing the metal sheet or strip.

Yet another objective of the invention is to provide a CRM in which less stress remains on the pairs of working rolls.

Yet another objective of the invention is to provide better strip shape.

Still another objective of the invention is to provide a CRM in which less heat is generated, and a requirement of a heat exchanger in the CRM is eliminated.

Still another objective of the invention is to provide a CRM that requires less electrical power for its operation.

SUMMARY OF THE INVENTION

This summary is provided to introduce aspects related to a CRM, and the aspects are further described below in the detailed description. This summary is not intended to identify essential features of the claimed subject matter nor is it intended for use in determining or limiting the scope of the claimed subject matter.

In one embodiment, the CRM comprises a pair of working rolls configured to apply stress on a metal strip for reducing thickness of the metal strip having a width of 1250 mm. Face width of the pair of working rolls is 100 mm to 120 mm greater than a strip width. The CRM further comprises a pair of intermediate rolls configured to provide mechanical support to the pair of working rolls. Face width of the pair of intermediate rolls is 30 mm to 50 mm greater than the strip width. The CRM further comprises a pair of back-up rolls configured to provide mechanical support to the pair of intermediate rolls. Face width of the back-up rolls is 50 mm to 70 mm greater than the strip width.

In one embodiment, the CRM may be a 4-Hi CRM or a 6-Hi CRM. Further, bearing center distance of the CRM (200) is 2170 mm. The CRM transfers more stress on the metal sheet or strip and less stress remains on the pairs of rolls, thereby causing greater reduction in thickness of the metal sheet strip with application of similar Roll Separating Force (RSF).

Other aspects and advantages of the invention will become apparent from the following description, taken in conjunction with the accompanying drawings, illustrating by way of example the principles of the invention.

BRIEF DESCRIPTION OF THE DRAWINGS

The accompanying drawings constitute a part of the description and are used to provide a further understanding of the present invention.

FIG. 1 illustrates a side view of a conventional CRM pressing a metal strip, in accordance with prior art.

FIG. 2 illustrates a front view of proposed CRM having preferred dimensions, in accordance with an embodiment of the present invention.

FIG. 3a illustrates stress distribution across a metal strip and working rolls processing the metal strip, in accordance with an embodiment of the present invention.

FIG. 3b illustrates strip stress analysis in proposed CRM, in accordance with an embodiment of the present invention.

FIG. 4 illustrates thermal and mechanics analysis data for 1250 mm wide strip processed by proposed CRM operated at a speed of 800 MPM, in accordance with an embodiment of the present invention.

FIG. 5 illustrates thermal and mechanics analysis data for 1250 mm wide strip processed by proposed CRM operated at a speed of 1200 MPM, in accordance with an embodiment of the present invention.

FIG. 6 illustrates thermal and mechanics analysis data for 1300 mm wide strip processed by proposed CRM operated at a speed of 550 MPM, in accordance with an embodiment of the present invention.

DETAILED DESCRIPTION OF THE INVENTION

The detailed description set forth below in connection with the appended drawings is intended as a description of various embodiments of the present invention and is not intended to represent the only embodiments in which the present invention may be practiced. Each embodiment described in this disclosure is provided merely as an example or illustration of the present invention, and should not necessarily be construed as preferred or advantageous over other embodiments. The detailed description includes specific details for the purpose of providing a thorough understanding of the present invention. However, it will be apparent to those skilled in the art that the present invention may be practiced without these specific details.

Current disclosure provides a roll profile design to achieve significant reductions on a CRM to produce much lighter gauges in the least number of passes, without compromising on strip shape.

Roll bending and roll flattening are two key phenomenon involved in a cold rolling process. Rolls of a CRM act like beams as the separating force causes the rolls to bend and the amount of bending depends on size and the length of the rolls and width of a metal strip to be processed. In a 6-HI mill, intermediate rolls can be laterally shifted to roll virtually any width of strip with any incoming profile at any roll separating force, to nullify the roll bending movement, but roll bending cannot be eliminated. Roll flattening depends on total Roll Separating Force (RSF), diameter of the roll, and face width of the roll or the roll contact outside the strip edges.

With the market trends moving towards thinner gauge and high strength steels, it becomes imperative to optimize the cold rolling process to increase the reduction on the strip, without compromising the strip quality.

The instant application proposes CRMs using work rolls of barrel length ranging from x+100 to x+120, intermediate rolls of barrel length ranging from x+30 to x+50, and back-up rolls having barrel lengths ranging from of x+50 to x+70, while ‘x’ denotes strip width in millimeters. Barrel length of intermediate roll could be designed to accommodate minimum and maximum strip widths to be rolled. Such optimization of the barrel lengths optimized desirable same strip and roll width feature of the CRMs. Reduction in barrel lengths led to reduction of bearing center, i.e., loading points which helped to improve the strip profile significantly.

Referring to FIG. 2, a 6-Hi CRM 200 of preferred dimensions is described. The CRM 200 comprises a pair of working rolls 202 configured to apply stress on a metal strip for reducing thickness of the metal strip of width 1250 mm. The pair of working rolls 202 have a face width/barrel length of 1350 mm. A pair of intermediate rolls 204 is configured to provide mechanical support to the pair of working rolls 202. The pair of intermediate rolls 204 have a face width/barrel length of 1280 mm. Further, a pair of back-up rolls 206 is configured to provide mechanical support to the pair of intermediate rolls 204. The back-up rolls 206 have a face width/barrel length of 1300 mm. Bearing center distance of the CRM 200 is 2170 mm.

In one implementation, a simulation test was conducted using SOLIDWORKS® Simulation Premium platform, for performing stress analysis of the CRM 200.

Specifically, the simulation test was conducted to analyze effect of the roll profile and load centers on actual stress distribution on the metal strip and the working rolls 202. The simulation test revealed the following results in Table 1 below.

TABLE 1 Analysis Type Static Mesh Type Solid Mesh Mesher Used Blended curvature-based mesh Jacobian Points 16 Maximum Element Size 80 mm Minimum Element Size 16 mm Mesh Quality Plot High Solver Type Direct Sparse Solver Soft Spring On

During the simulation test, a total load of 250,000 Kgf×4, i.e., 1,000,000 Kgf was applied on the 1250 mm wide and 0.5 mm thick metal strip on two models. One model was based on conventional roll profile used in 6-HI CRM and other model was based on the proposed design of 6-HI CRM 200.

FIG. 3a illustrates stress distribution across the metal strip and the working rolls 202. Curve 302 provides stress distribution on a metal strip processed using conventional CRMs. Curve 304 provides stress distribution on a similar metal strip processed using proposed CRM 200. Curve 306 provides stress distribution on a working roll of conventional CRMs. Curve 308 provides stress distribution on the working rolls 202 of the proposed CRM 200. From the curves 302 and 304, it is evident that the stress distribution across the metal strip on the proposed CRM 200 is quite uniform. It is also notable to observe that there is sharp edge drop-off on the conventional mill due to more bending movement of working rolls. FIG. 3b illustrates strip stress analysis conducted on the conventional CRMs and the proposed CRM 200. Block 320 corresponds to strip stress in a conventional CRM and block 322 corresponds to strip stress in the proposed CRM 200. The stress distribution illustrated in FIG. 3a indicates that stress on the working rolls 202 of the proposed CRM 200 is lower, i.e., the working rolls 202 are subject to lower mechanical and thermal fatigue.

The strip stress analysis shown in FIG. 3b suggests that optimization of the roll profile results in 15% more stress on the metal strip, and thus reduction in number of passes of the metal strip through the CRM, and a uniform stress distribution across width of the metal strip, which would ensure a better strip profile.

Multiple tests were also conducted for performing thermal analysis and observing mechanics of the CRM 200. FIG. 4 illustrates thermal and mechanics analysis data for 1250 mm wide strip processed by the CRM 200 operated at a speed of 800 MPM. FIG. 5 illustrates thermal and mechanics analysis data for 1250 mm wide strip processed by the CRM 200 operated at a speed of 1200 MPM. FIG. 6 illustrates thermal and mechanics analysis data for 1300 mm wide strip processed by the CRM 200 operated at a speed of 550 MPM. From data illustrated in FIGS. 4 through 6, it could be observed that energy going in to strip reduction in CRM 200 was between 50 to 80 percent compared to 48 to 59 percent in conventional CRMs.

Also, energy going in to the friction and conduction in the CRM 200 is significantly lower compared to conventional CRMs. The more the reduction in energy going in to the strip implies higher strip temperature. Such increase in strip temperature will cause decrease in material yield stress which is result of the increase in strain rate. Further, more energy going in to the strip means more reduction on the strip and hence, less number of passes are required to obtain final thickness from similar input thickness. It is to be noted that more number of passes result in higher yield stresses in the strip and greater conduction occurs between the strip and the rolls. Hence, the energy going in to the strip is less than the reduction energy and half the friction energy. The more energy going in to the strip means less rise in roll coolant temperature, and thereby eliminating the heat exchanger used for roll coolant cooling.

As the strip thickness becomes smaller, flattening of the rolls and axial bending of the rolls become proportionally larger. At some stage, depending on roll force, strip width, roll crown, and roll bending force, the top and bottom work rolls will come in contact outside the strip edges. A higher roll force has a tendency to make the strip shape poorer but the edge contact force in the work rolls plays an important role in thin strip rolling process. In case the edge contact force between two work rolls is controlled between 11 to 15 percent, it can improve the strip shape. During roll flattening, force applied to the strip will approach a fixed ratio of the total roll separating force and can be controlled to improve the strip profile.

In view of the above provided embodiments and their explanations, it is evident that the present invention offers:

-   -   12-15% more reduction on the strip and hence lesser number of         passes are required to achieve a desired thickness of the strip;     -   lesser yield stresses on the strip due as a result of the         reduced number of passes;     -   better strip shape profile on account of controlled work roll         edge contact force;     -   reduced thermal and mechanical fatigue on working rolls;     -   elimination of cooling arrangement for emulsion system; and     -   2-3% reduction in rolling power consumption.

Although implementations of CRMs have been described in language specific to structural features and/or methods, it is to be understood that the appended claims are not necessarily limited to the specific features or methods described. Rather, the specific features are disclosed as examples of the CRMs. 

1. A Cold Rolling Mill (CRM) (200) comprising: a pair of working rolls (202) configured to apply stress on a metal strip for reducing thickness of the metal strip, wherein a face width of the pair of working rolls (202) is 100 mm to 120 mm greater than a width of the metal strip; a pair of intermediate rolls (204) configured to provide mechanical support to the pair of working rolls (202), wherein a face width of the pair of intermediate rolls (204) is 30 mm to 50 mm greater than the width of the metal strip; and a pair of back-up rolls (206) configured to provide mechanical support to the pair of intermediate rolls (204), wherein a face width of the pair of back-up rolls (206) is 50 mm to 70 mm greater than the width of the metal strip.
 2. The CRM (200) as claimed in claim 1, wherein the CRM (200) is a 4-Hi CRM or a 6-Hi CRM.
 3. The CRM (200) as claimed in claim 2, wherein the width of the metal strip is 1250 mm.
 4. The CRM (200) as claimed in claim 1, wherein a bearing center distance of the CRM (200) is 2170 mm.
 5. The CRM (200) as claimed in claim 1, wherein the CRM (200) is configured to transfers more stress on the metal strip such that less stress remains on each of the pairs of rolls (202, 204, 206) resulting in better strip shape and greater reduction of thickness of the metal strip.
 6. A method of reducing thickness of a metal strip using a Cold Rolling Mill (CRM) (200), the method comprising: applying stress on the metal strip via a pair of working rolls (202) of the CRM; mechanically supporting the pair of working rolls (202) via a pair of intermediate rolls (204) of the CRM (200); and mechanically supporting the pair of intermediate rolls (204) via a pair of back-up rolls (206), wherein a face width of the pair of working rolls (202) is 100 mm to 120 mm greater than a width of the metal strip.
 7. The method as claimed in claim 6, wherein a face width of the pair of intermediate rolls (204) is 30 mm to 50 mm greater than the width of the metal strip.
 8. The method as claimed in claim 6, wherein a face width of the pair of back-up rolls (206) is 50 mm to 70 mm greater than the width of the metal strip.
 9. The method as claimed in claim 6, wherein the width of the metal strip is 1250 mm.
 10. The method as claimed in claim 6, wherein a bearing center distance of the CRM (200) is 2170 mm.
 11. The method as claimed in claim 6, wherein more stress is applied on the metal strip such that less stress remains on each of the pairs of rolls (202, 204, 206).
 12. The method as claimed in claim 6, wherein the CRM (200) is a 4-Hi CRM or a 6-Hi CRM. 